Sammendrag
NIVA and FREVAR conducted a monitoring survey of the water quality in Lake Vestvannet and Lake Borredalsdammen in Fredrikstad in 2016, with focus on planktonic algae and cyanobacteria. The findings are compared to data from previous years. Norwegian Food Safety Authoritys guidance together with NIVAs proposition for suitability criteria for drinking water (Solheim et al. 2008) is taken into consideration in the discussions of the results. In addition, the most recent guidance (Veileder 02:2013 Direktoratsgruppa, 2013) for ecological classification of waters is used as a tool in this report. The content of the nutrients nitrogen and phosphorus have decreased in 2016 compared to 2015. The algal biomass in both lakes have decreased in 2016, while the chlorophyll quantity was approximately the same as last year. There are no observed significant changes in any parameters measured in short term or long term, nor in algal composition. Most of the algal community consists of species that are common in lakes in Østfold county, and these are not normally toxin producers. However, a small amount of microcystin was measured throughout the season in both lakes. The biomass of cyanobacteria is generally low in both lakes, and the microcystin level measured was well below the limits for both drinking water supplies and bathing water. Lake Vestvannet is classified to high ecological status by the Water Framework Directive guidance, and both lakes are considered to be well suitable for drinking water with regards to microcystin. However, both are considered poorer with regards to phosphorous contents and chlorophyll levels.
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